1. Field of the Invention
The present invention relates generally to collet chuck assemblies and particularly to a precision, true length, collet chuck assembly for holding a workpiece against axial displacement during the collet closing and opening sequences, and which utilizes collets that can be quickly changed from one collet size to another.
2. The Prior Art
xe2x80x9cQuick changexe2x80x9d collets for mounting on the spindles of machine tools such as automatic screw machines, lathes, and the like, are well known. Examples of such collets, manufactured by Hardinge, Inc., Elmira, N.Y., are disclosed in U.S. Pat. Nos. 4,858,938 and 5,096,213, both of which are incorporated by reference herein in their entireties. A typical quick change collet has two parts including a collet head and a collet body for receiving the collet head. The collet head comprises a segmented structure having an internal, typically cylindrical work gripping surface and an external, outwardly flaring surface adapted to cooperate with a correspondingly flared inner surface formed on a mounting fixture bolted to the spindle of the machine tool. The rear portion of the collet head includes a radially outwardly projecting flange that locks with a radially inwardly extending ridge on the collet body. Removal of the collet head is accomplished by pressing together the segments of the collet head at the rear portion by means of a special tool so that the flange on the collet head clears the ridge on the collet body. The collet head is then easily withdrawn from the collet body. Installation of the collet head is accomplished by the same tool which compresses the collet head segments so that the flange on the collet clears the ridge in the collet body as the rear end of the collet head is slid into the tubular collet body. When the flange clears the ridge, pressure is released from the collet head allowing the collet head to expand and the flange to lock with the ridge. A tool for installing the collet head in the collet body and removing it therefrom is disclosed in U.S. Pat. No. 5,087,059, incorporated herein by reference in its entirety.
The segments of the segmented collet head are spaced apart circumferentially and connected together by means of rubber inserts spanning the spaces between adjacent segments. The rubber inserts function to hold the segments of the collet head together while at the same time providing a spring means to force the collet head segments to move apart when closing pressure is removed from the collet head to release the workpiece. When the collet head is in the closed or work gripping position, the rubber inserts are substantially compressed.
The rear portion of the collet body is adapted to threadedly receive a draw or pull tube forming part of the machine tool. With the collet head in place in the collet body, retraction of the pull tube retracts the collet body and the collet head attached thereto relative to the mounting fixture so as to compress the collet head and grip the workpiece. Conversely, advancement of the pull tube pushes the collet body and attached collet head forwardly relative to the mounting fixture permitting the collet head to open and release the workpiece. Because the work-holding collet head moves relative to the fixed mounting fixture of the machine tool, this longitudinal or axial displacement can cause errors in the machining operation.
True or dead length collet chuck assemblies for holding a standard, that is, non-quick change, collet against longitudinal or axial displacement so as to eliminate errors in the machining operation are also well known. For example, U.S. Pat. Nos. 5,480,164 and 5,855,377, both of which are incorporated herein by reference in their entireties, each discloses a true length collet chuck assembly in which a standard collet having a split forward end portion is threadedly received by a collet retaining ring secured to an interior surface of a chuck body. A longitudinally displaceable camming or closing sleeve surrounding the collet and having an outwardly flaring, internal camming surface operatively associated with an outwardly flaring, exterior surface on the split forward end portion of the collet, compresses or releases the jaws of the collet. The longitudinally displaceable closing sleeve is actuated by means of a collet actuator coupled to the closing sleeve and to a mechanically or hydraulically operated push tube mounted on the machine tool. Advancement or forward axial movement of the closing sleeve relative to the axially fixed collet compresses the jaws of the collet to grip the workpiece while retraction of the closing sleeve releases the collet jaws from the workpiece.
Although the true length collet chucks of the aforementioned patents substantial advanced the state of the art, there continues to exist a need for still further improvements. For example, it would be desirable to provide a collet chuck assembly that can provide the advantages of both true length and quick change collet operation.
In accordance with one, specific exemplary embodiment of the present invention, there is provided a true length collet chuck assembly having a longitudinal, central axis. The collet chuck assembly is adapted to receive and actuate a quick change collet having a longitudinal, central axis that coincides with the axis of the assembly when the collet is in place within the assembly. The quick change collet defines a radially extending rear face, an inner workholding surface concentric with the axis of the collet and an outer surface concentric with the axis of the collet and including an outwardly flaring surface portion. The collet further includes a flange extending radially outward from the outer surface of the collet adjacent the rear face of the collet.
The collet chuck assembly comprises a chuck body having a longitudinally extending bore centered on the axis of the assembly and defined by an inner cylindrical surface on the chuck body. The inner cylindrical surface on the chuck body defines a circumferential groove for receiving the radially outwardly extending flange on the quick change collet to thereby restrain the collet against axial movement relative to the chuck body. The assembly further includes a collet closing sleeve disposed within the chuck body and axially displaceable relative thereto. The closing sleeve has front portion, a rear portion and an interior surface, the interior surface of the closing sleeve at the front portion thereof being outwardly flared to engage the outwardly flared surface on the quick change collet. Axial displacement of the closing sleeve relative to the chuck body in one direction compresses the collet to grip a work piece and axial displacement of the closing sleeve in the other direction allows the quick change collet to expand to release the work piece. The collet chuck assembly also includes a collet actuator disposed within the chuck body, the collet actuator being adapted to be coupled to a collet actuator driver, such as a push tube, for axially displacing the collet actuator relative to the chuck body. The collet actuator is coupled to the rear portion of the closing sleeve, whereby axial displacement of the collet actuator correspondingly displaces the closing sleeve whereby the collet is compressed or allowed to expand.
In accordance with another aspect of the specific, exemplary embodiment of the invention, the collet closing sleeve includes a plurality of rearwardly extending fingers, the collet actuator and the fingers on the collet closing sleeve being configured to interengage so that they are movable together axially. The chuck body preferably includes a radially inwardly extending wall defining the inner cylindrical surface on the chuck body, the front portion of the closing sleeve being disposed forwardly of the wall and the collet actuator being disposed rearwardly of the wall. The wall defines a plurality of longitudinally extending, circumferentially spaced apart openings, each of the plurality of rearwardly extending fingers on the collet closing sleeve extending longitudinally through one of the openings. Further, each of the plurality of rearwardly extending fingers on the collet closing sleeve has an internal surface defining a circumferentially extending groove. The collet actuator includes a plurality of circumferentially spaced apart, radially outwardly projecting flange segments, each flange segment being received by the groove of a corresponding finger in interlocking fashion. The circumferential spaces between the flange segments on the collet actuator each subtends a first angle while each of the plurality of rearwardly extending fingers on the collet closing sleeve subtends a second angle. The first angle is greater than the second angle so as to permit separation of the collet closing sleeve and the collet actuator. Preferably, both the collet closing sleeve and the collet actuator are releasably restrained against rotation relative to the chuck body.
The quick change collet may be installed in the chuck body and removed therefrom in a manner well known in the art using a special tool, facilitating the rapid interchangeability of collets having various sizes. At the same time, the restraint of the installed quick change collet relative to the chuck body in the longitudinal or axial direction provides a true length feature for precise workpiece length control.